Frequency modulated oscillator control



June 28, 1949.

R. H. RANGER FREQUENCY MODULATED OSCILLATOR CONTROL Filed Oct. 2, 1944/NVEA/rof? Patented June 28, 1949 UNITED ISTATES PATENT OFFICE (Grantedunder the act .of March `3, 1883, `as amended April 30, 1928; 370 O. G.`757) l The invention :described herein may 'be manufactured .and usedYbyor for the Government for governmental purposes, without'thc payment0f 'any royalty thereon.

This invention relates to means for keeping the output frequency of anelectric oscillator substan- `tially constant.

An object of this invention is to lprovide a frequency control in `whicha frequency-modulated oscillator is employed as a reference lor basis.

More specilically, it is .an additional object of 1my invention toafford a frequency controller comprising a crystal-controlled oscillatorwhich provides frequencies `of great stability, a controlled oscillatorcontaining an adjustable part by inovemen't of which its outputfrequency is varied, a frequency-modulated oscillator having a limitedyrange of output frequencies, a source of frequencies providing saidfrequency-modulation, a 'first mixer-detector lin which the outputfrequencies of said crystal-controlled oscillator and of said controlledoscillator are combined, a second mixer-detector'in whichthe outputy'from said first mixer-detector andthe output fromsaidVfrequency-modulated oscillator are combined in such 4phase relationshipthat the output frequency of said second mixer-detector is in phase withthe frequency outputof said source when the output of -the firstmixer-detector is of higher frequency than that of thefrequency-modulated oscillator, f-

'and that the output yfrequency of said second :mixer-detector is 180-outof phase `with the `frequency output of rsaid source when the outputof `the mixer-detectorisof'lower frequency than fthat of thefrequency-modulated oscillator, a

fspl-it-pl;1ase motor having coils energized respectively :by theAoutput of said source and by the output of said second mixer-detector,and a me` fchanical connection betweenfsaid'motor and said adiustablepart whereby vsaid vmotor moves vsaid `same automatic 'volumecontrolmeans are vso ar- :ranged as to decrease tthetfrequency swing'of the.frequency-modulated.oscillator circuit as the frelquency of thecontrolled oscillator approaches its desired value.

I vhavedescribed.and illustrated a preferred embodment of my inventionin the annexed specification and drawing to which `reference is nowmade.

The single figure of the drawing is a block diag-ram.

We start with-acrystal l as seen in thedrawing. Let it he assumed thatthis is a 200 kc. crystal. The harmonic .amplifier .2 will then build up.the output of this crystal .to give rich harmoniccontent. Harmonicselector 3 will pick out the particular harmonic desired Afor reference.The conv.trolled oscillator .L4 .at the bottom will pass some .of itsenergy up ,into the .rst mixer-detector 5 .along with theroutput.frequency of the harmonic selector y3 and give a iirst beat, .constantin fre- .quency The output yof the iirstmixer-detector y5 is combinedwith .that of anadjustable oscillator 6 cover- .ing a band offrequencies from 200 to 4001kc. If .the frequency of the .controlledoscillator .I4 is 290 kc. .away from the selected harmonic in 3, WeWillhave a `200 kc. beat-coming out of the first mixer-detector.

If this is vcombined with .a ,200 kc. setting of `the oscillator 6 .wewould get 0 beat in the-second mixeradetector l. However, this, ofcourse, would only be by chance. Let it be assumed that the Yfirstmixer-detector ,5 is putting outa frequency of 2.1.0 kc. We will thenhave a difference frequency at second mixer-detector 'I of 10 kos.between the 20G kc. setting `of oscillator 6 and the output of the.first mixer-.detector f5.

Now assume that we have a (S0-cycle source 'shown at 18. This 60 cycleWorks through a gain control i6 to give a mechanical vibration to theplates of thetrimmer condenser 2li connected to oscillator Ci. Asis-well known, the gain control I6 vmay be caused to impart thisvmechanical vibration "to the Vplates of condenser 20 by feeding out-Iput of gain control i6 to an electromechanical translating device y2lwhich may be of a convenf'tional type -as an lelectrodynamic or magneticloudspeaker havingits armature connected either ydirectly or throughsome form of mechanical coupling to the condenser plates. Commonsolenoids may also be-used fas the translating devices. The net resultwill beto give la frequency moduilationto the output of this oscillator6 at the rat-e of 60 cycles. vhet it be assumed that 'the phases :aresuch that Lfor -a rising .60 'cycle pulse, the fre- 55 iquenoy-of the'oscillator i6 is raised. `We will then have an output going over to thesecond mixerdetector 'I which will give with the 210 kc. coming from thefirst mixer-detector 5 a second wobbled beat as the output of 1. As thefrequency put out by the local oscillator 6 rises, it will approach thatof the beat coming out of mixer-detector 5 so that we will get areduction in frequency in the output of the second mixer-detector I onpositive y swing. In other words, the frequency will decrease onpositive swing when the local oscillator frequency is below that of thebeat frequency coming from 5. On the contrary, if the output of thefirst mixer-detector 5 was below 20() kc., say 190 lacs., the frequencyof the output of mixerdetector I would rise on positive swings given thelocal oscillator So we have here a complete reversal in action,depending upon whether the rst mixer-detector output is below or abovethat of the local oscillator 6. Capacity coupling 8 is introduced in theoutput of second mixer-detector T such that for higher frequencies, moreoutput will be obtained. Under these conditions, we will have passed onthrough gain control I1 into frequency modulation-discriminator 9 (whichmay include an amplifier), an alternating current at 60 cycles whichwill be in phase with the original (iO-cycle impulse I8 working upthrough the local oscillator, if the frequency of the firstmixerdetector output 5 is below that of the local oscillator. Converselyit will be out of phase with the (iO-cycle input I8, if the frequency ofthe first mixer-detector 5 is above the frequency of the localoscillator 6. The discriminator S, in conventional fashion, serves toeliminate the radiofrequency currents in its input and to confine itsoutput current to the same frequency as that of source i8, which in thisexample has been chosen as 60 cycles. The net result is that if We applythe same (iO-cycle output I8 into one of the coil windings of the splitphase motor Hl and use the output of the frequencymodulation-discriminator 9 (which may include an amplifier), to energizethe other coil winding of the split phase motor Il), we will have aresultant motion in one direction or the other, depending on whether theoutput of the first mixer-detector 5 is above or below the frequency ofthe swinging local oscillator 5. The motor ID is of the well-knownsplit-phase type, with the minor variation that the connections to themain and auxiliary windings, respectively, are not interconnected butthe connections for these respective windings are brought out of themachine separately, thus making it possible so to connect the respectivewindings to their inputs as to provide the desired direction of rotationof the motor. The connections for one of these windings derive inputfrom source I8, while the connections for the other winding derive inputfrom frequency-modulation-discriminator 9. As described in any textbookon alternating-current machinery, (see, for example, pages 474 and 475of Electrical Circuits and Machinery, vol. II, Alternating Currents, byHehre Harness, 1942 addition, published by yJohn Wiley & Sons, Inc.) themotor will then operate. The impedance of the auxiliary winding is suchthat the current in this winding has sufficient phase lag, with respectto the current in the main winding, to produce a rotating iield in themotor. When the two input voltages are in phase, the motor will rotatein one direction. A reversal in phase of the input derived fromdiscriminator 9 will cause the motor to rotate in Athe reversedirection.l Of course, the motor will iz. not operate unless it obtainsinput from both source I8 and discriminator 9.

Mechanical connections II are taken from the split-phase motor I0 to thecontrolled oscillator le, working on the adjustable condenser capacitorI2. Mechanical connections are tied in with the phase control such thatif the frequency of the controlled oscillator is less than 200 kcs.below the selected harmonic 3, then a correction will be applied tocapacitor I2 such as to decrease its capacity and thereby increase thefrequency of the controlled oscillator set by capacitor I3.

Automatic volume control is introduced, as shown on I5, which Works offcapacity coupling 8 on the output of the second mixer detector 1, suchthat on rectication of the output of this capacity coupling 8, goinginto automatic volume control I5, a source of current will be obtainedproportional to the intensity of the 60 cycle beat at this point.Automatic volume control i5 may, if desired, be coupled to the output ofSecond mixer detector l independently of capacity coupling 8. If this 60cycle beat is lower in intensity, the operation of the automatic volumecontrol i5 will be such as to increase the gain at I'I to give morepower to drive the split-phase motor.

It will also be arranged to decrease the gain control iii so that lessfrequency swing will be accomplished on the local oscillator B. The netresult of these operations is such that as the output of the first mixerdetector 5 approaches the setting of the local oscillator E, we will geta lower output in the second mixer detector, which will be stepped upagain through the gain control il and the discriminator 9 to keepdriving the split-phase motor and at the same time the gain control I6will decrease the frequency swing of oscillator 6 so that the netadjustment will come closer and closer to the desired frequency as thisaction continues to bring it near the required point.

The output of the controlled oscillator is taken to a transmitter whichis not run at the same frequency, but either twice the frequency oronehalf the frequency of the controlled oscillator i4 so that there willbe less chance of interaction between the nal output stages of thetransmitter and the controlled oscillator stages.

It is obvious that this whole arrangement may be worked to control anypoint from 200 to 400 kc. on the local oscillator E as any other pointfrom 200 to 400 may be taken such as 230 kc. as the reference note tocompare with the difference between the frequencies of the controlledoscillator I4 and the selected harmonic In this case, the controlledoscillator I4 will be locked to a point 230 kc. away from the selectedharmonic.

Continuous control of crystal accuracy will thus be realized, at anyselected point 200 to 400 kc. away from the selected harmonic, and theaccuracy of the local oscillator 6 will only have to be arithmeticallyaccurate to insure overall splitphase frequency control of the entireoutfit.

The specification frequencies which are referred to in connection withthe above description are merely illustrative and the system may be usedwith other frequencies of the crystalcontrolled oscillator circuit, witha different range of possible frequency adjustment for thefrequency-modulated oscillator circuit, and with a different frequencyof the source of frequencies providing frequency-modulation.

Although two mixer detectors have been referred to above, it would ofcourse, be possible to 5 combine the functions of both of these in` asingle tube, if desired.

I claim:

l. Means for maintaining constant the output frequency of an oscillator,said means comprising a controlled oscillator whose output frequency isto be maintained constant, an adjustable p-art of said oscillator whoseadjustment varies the output frequency thereof, a split phase motorjoined to said part so that said motor and said part move together, asource of relatively low frequencies, a second oscillator having arelatively constant output frequency which normally has a predetermineddifference from the frequency of the controlled oscillator, a thirdoscillator including a movable part controlled by said source and whosemovements cause frequencymodulation of said third oscillator andincluding also means for adjusting said third oscillator to have a meanfrequency which is equal to the above predetermined. difference, anelectric circuit connection between said source and said motor, anelectric circuit connection between all three oscillators and saidmotor, whereby said motor moves said adjustable part and therebycorrects the frequency of said controlled oscillator in accordance withwhether or not the difference between the output frequency of saidcontrolled oscillator and of said second oscillator is above or belowthat of said third oscillator.

2. Means for maintaining constant the output frequency of an oscillator,said means comprising a controlled oscillator Whose output frequency isto be maintained constant, an adjustable part of said oscillator whoseadjustment varies the output frequency thereof, a split phase motorjoined to said part so that said motor and said part move together, asource of relatively low frequencies, a second oscillator having arelatively stable output frequency which normally has a predetermineddifference from the frequency of the controlled oscillator, a thirdoscillator including a movable part controlled by said source and whosemovements cause frequency-modulation of said third oscillator andincluding also means for adjusting said third oscillator to have a meanfrequency which is equal to the above predetermined difference, a firstmixer-detector in which the output frequencies of said controlledoscillator and of said second oscillator are combined to give an outputequal to any difference between these frequencies, a secondmixer-detector in which the output frequencies of said firstmixer-detector and of said third oscillator are combined in such phaserelation that the output of said second 'T mixer-detector remains inphase with or is shifted 180 out of phase with the output of said sourcedepending upon whether the frequency of said third oscillator is belowor above that of said first mixer-detector, an electric circuitconnection between said source and said motor, an electric circuitconnection between said second mixer-detector and said motor wherebysaid motor moves said adjustable part in one direction or the otherdepending upon the phase relations of the outputs of said source and ofsaid second mixerdetector fed to said motor and thereby corrects thefrequency of said controlled oscillator.

3. Means for maintaining constant the output frequency of an oscillator,said means comprising a controlled oscillator whose output frequency isto be maintained constant, an adjustable part of said oscillator whoseadjustment varies the output frequency thereof, a split phase motorjoined to said part so that said motor andsaid part move together, asource of relatively low frequencies, a second oscillator having arelatively stable outputr frequency which normally has a predetermineddilference from the frequency of the controlled oscillator, a thirdoscillator including a movable part controlled by said source and whosemovements cause frequency-modulation of said third oscillator andincluding also means for adjusting said third oscillator to have a meanfrequency which is equal to the above predetermined difference, anelectric circuit connection between said source and said motor, capacitycoupling electrically connected in circuit to receive out put from allthree oscillators and arranged to modify the volume of its own output inproportion to the frequency of its input, an electric circuit connectionbetween said capacity coupling and said motor, whereby said motor movessaid adjustable part and thereby corrects the frequency of saidcontrolled oscillator in accordance with whether or not the differencebetween the output frequency of said controlled oscillator and of saidsecond oscillator is above or below that of said third oscillator.

4. Means for maintaining constant the output frequency of an oscillator,said means comprising a controlled oscillator whose output frequency isto be maintained constant, an adjustable part of said oscillator whoseadjustment varies the output frequency thereof, a split phase motorjoined to said part so that said motor and said part move together, asource of relatively low frequencies, a second oscillator having arelatively stable output frequency which normally has a predetermineddifference from the frequency of the controlled oscillator, a thirdoscillator including a movable part controlled by said source and whosemovements cause frequency-modulation of said third oscillator andincluding also means for adjusting said third oscillator to have a meanfrequency which is equal to the above predetermined difference, anelectric circuit connectionbetween said source and said motor,frequency-modulation-discriminator means electrically connected incircuit to receive output from all three oscillators and arranged topass and amplify current at the frequency of said source, and anelectric circuit connection between said discriminator means and saidmotor, whereby said motor moves said adjustable part and therebycorrects the frequency of said controlled oscillator in accordance withwhether or not the difference between the output frequency of saidcontrolled oscillator and of said second oscillator is above or belowthat of said third oscillator.

5. Means for maintaining constant the output frequency of an oscillator,said means comprising a controlled oscillator whose output frequency isto be maintained constant, an adjustable part of said oscillator whoseadjustment varies the output frequency thereof, a split phase motorjoined to said part so that said motor and said part move together, asource of relatively low frequencies, a second oscillator having arelatively stable output frequency which normally has a predetermineddifference from the frequency of the controlled oscillator, a thirdoscillator including a movable part controlled by said source and whosemovements cause frequency-modulation. of said third oscillator andincluding also means for adjusting said third oscillator to have a meanfrequency which is equal to the above predetermined difference, anelectric circuit connection between said source and said motor, capacitycoupling electrically connected in circuit to receive output from allthree oscillators and arranged to modify the volume of its own output inproportion to the frequency of its input, automatic volume control meanselectrically connected to the output of said capacity coupling andmodifying in proportion to the extent of any departure of the frequencyof the controlled oscillator from its desired constant value, the extentof the frequency swing given by said source to said third oscillator, anelectric circuit connection between said capacity coupling and saidmotor, whereby said motor moves said adjustable part and therebycorrects the frequency of said controlled oscillator in accordance withwhether or not the difference between the output frequency of saidcontrolled oscillator and of said second oscillator is above or belowthat of said third oscillator.

5. Means for maintaining constant the output frequency of an oscillator,said means comprising a controlled oscillator whose output frequency isto be maintained constant, an adjustable part of said oscillator whoseadjustment varies the output frequency thereof, a split phase motorjoined to said part so that said motor and said part move together, asource of relatively low frequencies, a second oscillator having arelatively stable output frequency which normally has a predetermineddifference from the frequency of the controlled oscillator, a thirdoscillator including a movable part controlled by said source and whosemovements ycause frequency-modulation of said third oscillator andincluding also means for adjusting said third oscillator to have a meanfrequency which is equal to the above predetermined difference, anelectric circuit connection between said source and said motor, capacitycoupling electrically connected in circuit to receive output from allthree oscillators and arranged to modify the volume of its own output inproportion to the frequency of its input, automatic volume control meanselectrically connected to the output of said capacity coupling andmodifying in inverse proportion to the intensity of that component ofsaid output which has the modulation frequency, the volume of the powersupplied said motor, and an electric circuit connection between saidcapacity coupling and said motor, whereby said motor moves saidadjustable part and thereby corrects the frequency of said controlledoscillator in accordance with whether or not the difference between theoutput frequency of said controlled oscillator and of said secondoscillator is above or below that of said third oscillator.

'7. A frequency controller comprising: an oscillator which provides anoutput having great stability of frequency; a controlled oscillatorwhose output has a frequency which normally has a predetermineddifference from the frequency of the output of the first-mentionedoscillator, such controlled oscillator containing an adjustable part bymovement of which its output frequency is varied; a frequency-modulatedoscillator capable of adjustment so that its output has a mean frequencywhich is equal to the above predetermined difference; means forfrequency-modulating the frequency-modulated oscillator; a firstmixer-detector in which the outputs of said firstmentioned oscillatorand of said controlled oscillator are combined; a second mixer-detectorin which the outputs of said first mixer-detector and of saidfrequency-modulated oscillator are combined in such phase relationshipthat the output of said second mixer-detector is in phase with theoutput of said frequency-modulating means when the output of the rstmixer-detector is of higher frequency than that of thefrequency-modulated oscillator and is 180 out of phase with the outputof said frequency-modulating means when the output of the firstmixer-detector is of lower frequency than that of thefrequency-modulated oscillator; a split-phase motor having coils,energizecl respectively by the output of said frequency-modulating meansand by the output of said second mixer-detector; and a mechanicalconnection between said motor and said adjustable part, whereby saidmotor moves said adjustable part to bring the frequency of the output o1said controlled oscillator to the desired value.

8. A frequency controller according to claim '7, h a v i n g frequencymodulation discriminator means connected between said secondmixerdetector and one of the coils of said split-phase motor.

9. An electrical frequency control system comprising: a first oscillatorcircuit; a controlled oscillator circuit which includes tuning means andwhose frequency normally has a predetermined difference from thefrequency of the first circuit; a third oscillator circuit includingmeans for adjusting this circuit so to have a mean frequency which isequal to the above predetermined difference; means forfrequency-modulating the third circuit; mixer-detector means whichderive input from all three oscillator circ u i t s frequency modulationdiscriminator means which derive input from the mixer-detector means andwhich pass only current at substantially the modulation frequency of thethird oscillator circuit; and control means which derive input from thefrequency-modulation-discriminator means and from thefrequency-modulating means and which are responsive to a variation ofthe controlled frequency from its normal value, to operate the tuningmeans in the controlled circuit to bring the controlled frequency to itsnormal value.

10. An electrical frequency control system comprising a first oscillatorcircuit, a controlled oscillator circuit which includes tuning means andwhose frequency normally has a predetermined diiference from thefrequency of the first circuit, a third oscillator circuit includingmeans for adjusting this circuit so as to have a mean frequency which isequal to the above predetermined difference, means forfrequency-modulating the third circuit, a first mixer-detector derivinginput from the first oscillator circuit and from the controlledoscillator circuit, a second mixer-detector deriving input from thefirst mixer-detector and from the third oscillator circuit,frequency-modulation-discriminator means deriving input from the secondmixer-detector and passing only current at substantially the modulationfrequency of the third oscillator eircuit, and control means derivinginput from the frequency-modulation-discriminator means and from thefrequency-modulating means and responsive to a variation of thecontrolled frequency from its normal value, to operate the tuning meansin the controlled circuit to bring the controlled frequency to itsnormal Value.

1l. An electrical frequency control system7 as described in claim 10,which includes automatic volume control means deriving input from thesecond mixer-detector for modifying inverse proportion to the volume ofthat component of the input which has the modulation frequency, thevolume of that part of the input to the control means which is derivedfrom the frequencymodulation-discriminator means.

12. An electrical frequency control system, as described in claim l0,which includes automatic volume control means deriving input from thesecond mixer-detector for modifying the extent of the frequency swing ofthe third oscillator circuit in proportion to the difference of thecontrolled frequency from its normal value.

13. An electrical frequency control system, as described in claim 10,which includes automatic volume control means deriving input from thesecond mixer-detector for modifying in inverse proportion to the volumeof that component of the input which has the modulation frequency, thevolume of that part of the input to the control means which is derivedfrom the frequencymodulation-discriminator means, such automatic volumecontrol means also modifying the extent of the frequency swing of thethird oscillator circuit in proportion to the difference of thecontrolled frequency from its normal value.

14. An electrical frequency control system, as described in claim 10,which includes coupling means inserted between the second mixerdetectorand the frequency-modulation-dlscriminator means for modifying thevolume of the input to the latter means in proportion to the frequencyof the output of the second mixerdetector.

15. An electrical frequency control system, as described in claim 10,which includes coupling means inserted between the second mixer-detectorand the frequency-modulation-discriminator means for modifying thevolume of the input to the latter means in proportion to the frequencyof the output of the second mixer-detector, and also includes automaticvolume control means deriving input from the coupling means formodifying in inverse proportion to the Volume of that component of theinput which has the modulation frequency, the volume of that part of theinput to the control means which is derived from thefrequency-modulation-discriminator means.

16. An electrical frequency control system, as

means described in claim 10, which includes coupling inserted betweenthe second mixerdetector and the frequency-modulation-discriminatormeans for modifying the volume of the input to the latter means inproportion to the frequency of the output of the second mixerdetector,and also includes automatic volume control means deriving input from thecoupling means for modifying the extent of the frequency swing of thethird oscillator circuit in proportion to the difference of thecontrolled frequency from its normal value.

17. An electrical frequency control system as described in claim 10,which includes coupling means inserted between the second mixer-detectorand the frequency-modulation-discriminator means for modifying thevolume of the input to the latter means in proportion to the frequencyof the output of the second mixer-detector, and also includes automaticvolume control means deriving input from the coupling means formodifying in inverse proportion to the volume of that component of theinput which has the modulation frequency, the volume of that part of theinput to the control means which is derived from thefrequency-modulation-discriminator means, such automatic volume controlmeans also modifying the extent of the frequency swing of the thirdoscillator circuit in proportion to the difference of the controlledfrequency from its normal value.

RICHARD H. RANGER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,294,942 Varian Sept. 8, 19212,297,800 Read Oct. 6, 1942

